The present invention relates generally to surface acoustic wave (SAW) devices and more particularly to a saw device having improved performance characteristics for application to radio frequency (RF) filtering for wireless communications.
Surface acoustic wave (SAW) filters with resonant structures are used extensively for radio frequency (RF) filtering in wireless communication systems due to the small chip size and low insertion loss that can be realized in such filters. A high operating frequency requires a high propagation velocity of saw in a piezoelectric substrate, combined with strong piezoelectric coupling. In particular, rotated y-cuts of lithium niobate (LiNbO3) single crystals provide propagation velocities of surface waves up to 4700 m/s and electromechanical coupling coefficients k2 up to 17% if the propagation direction is parallel to x-axis of a crystal. Such orientations are characterized by the Euler angles (0xc2x0, xcexc, 0xc2x0). Therefore, these substrates are widely used in RF filters in spite of certain attenuation caused by a leaky nature of propagating surface waves. The attenuation of pseudo-surface (leaky) acoustic waves (PSAW) is usually minimized as a result of an appropriate choice of crystal orientation and thickness of metal electrodes.
The existing tendency to increase operating frequencies in communications systems is followed by an increasing interest in high velocity pseudo-surface waves (HVPSAW) with propagation velocities up to 7000 m/s. In general, HVPSAW are more strongly attenuated than low-velocity PSAW. With a proper choice of a substrate, orientation and electrode thickness propagation loss can be reduced. By way of example, low attenuated HVPSAW with propagation velocities of 6000-6500 m/s and a high electromechanical coupling coefficient can exist in rotated y-cuts of LiNbO3 with periodic grating of Al electrodes if the propagation direction lies in the YZ-plane of a crystal. Such orientations are described by the Euler angles (0xc2x0, xcex8, 90xc2x0). Reference can be made to Isobe (a. Isobe et al, IEEE trans. Ultrason., Ferroelect., Freq. Control, 1999, v.46, no 4, pp. 849-855) who reported that HVPSAW with negligible propagation loss exists in a 171xc2x0 YZxe2x80x2 cut of LiNbO3 with periodic grating of Al electrodes if a normalized thickness of electrodes is about 8.3% xcex9, where xcex9 is HVPSAW wavelength. This orientation can be characterized by the Euler angles (0xc2x0, 81xc2x0, 90xc2x0).
According to the theoretical characteristics of HVPSAW which propagate in a 171xc2x0 YZxe2x80x2 cut of LiNbO3 with Al grating of thickness 8.3% xcex9, as reported in Table 1 of the Isobe paper, a saw resonator shows maximum Q-factor at a resonant frequency, Qr=126,000. At an anti-resonant frequency, the calculated Q-factor was reported to be much smaller, Qa=3,840. From the description of experimental structure of saw resonators used by Isobe for Q-factor measurements, it follows that a metalization ratio (electrode width to period of grating structure) was assumed to be 0.5. In other words, Isobe et al. teaches a fixed metalization ratio and seeks a maximum Q-factor at a resonance frequency.
In many applications, the center frequency of a passband lies between resonant and anti-resonant frequencies of saw resonators that compose the filter structure. By way of example, resonant saw structures in ladder filters are used as both series and as parallel (shunt) components within a composite device structure, which may include lattice-like regions. In a typical ladder filter, the anti-resonant frequency of the parallel (shunt) elements and the resonant frequency of the series elements generally reside within the passband of the filter. In such a filter, the lower passband edge of the filter is shaped by the resonance of the parallel (shunt) elements and the upper passband edge is shaped by the anti-resonance of the series elements.
In view of the foregoing background, the present invention seeks to provide a piezoelectric substrate with an optimum crystal orientation for use in high radio frequency (RF) SAW devices, which can overcome the disadvantages of known substrate orientations. In addition, embodiments of the present invention as herein described by way of example, provide a filter with improved performance. Insertion loss is reduced and a shape factor improved for SAW filters comprising resonator-type elements that are built on an optimal orientation of LiNbO3 with Al grating of optimal thickness, by utilizing different metalization ratios in different resonator elements. Where different metalization ratios allow the propagation loss for each individual resonator to be minimized at predetermined individual frequencies, it is desirable to minimize the propagation loss of the shunt resonator elements that form the lower passband skirt at their resonant frequencies. It is also desirable to minimize the propagation loss at the anti-resonant frequency of the series elements that form the high frequency skirt of the series resonator elements. In yet other cases, it is desirable to minimize the loss of resonator elements at frequencies that result in minimizing the insertion loss of the filter.
The present invention may include a SAW device comprising resonator elements with improved performance due to utilizing orientations of LiNbO3 with simultaneously optimized propagation loss at resonant, anti-resonant, or frequencies in between, while the electrode thickness varies in a range from 8% xcex9 to 9% xcex9 (xcex9 herein defined as the acoustic wavelength). Further, the metalization ratio may be varied within the interval from 0.3 to 0.9.
The present invention may provide a SAW device comprising a piezoelectric substrate of a single crystal LiNbO3 with electrode patterns disposed on a surface of said piezoelectric substrate and forming resonator-type elements, wherein a thickness of electrode patterns are in the range from 8% to 9% xcex9 and Al is used as a primary component of electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (xcex, xcexc, xcex8), with angle xcex in the range from xe2x88x925xc2x0 to +5xc2x0, angle xcexc in the range from 75xc2x0 to 85xc2x0, and angle xcex8 in the range from 85xc2x0 to 95xc2x0. Further, a metalization ratio of the electrode patterns of resonator-type elements may be in the range from 0.3 to 0.9. Yet further, the metalization ratios of individual resonators may be varied to allow the propagation loss for each individual resonator to be minimized at predetermined individual frequencies.
In general, the metalization ratios of resonators, which shape the lower stop band edge, may be selected to minimize the propagation loss in the vicinity of the filter""s lower frequency skirt (i.e. at a resonant frequency of elements). Yet further, the metalization ratio of the resonator elements, which form the upper pass band skirt, may be selected to minimize the propagation loss in the vicinity of the upper stop band edge (i.e. at the anti-resonant frequency for the resonator). In cases of resonator elements, which do not primarily shape the pass band edges, it is desirable to minimize the loss of said resonator element at a frequency that will result in minimizing the insertion loss of the filter, wherein the magnitude of the variation between the values of the metalization ratios of the individual resonator elements exceeds 0.05.